A typical mercury wetted switch comprises a sealed glass capsule in which there is mounted a movable armature having a pair of pole ends for alternately contacting a respective electric contact when the armature is pivoting and the surface of the armature is wetted with a film of mercury. An energizing coil is provided to generate magnetic field in the magnetic conducting parts in order to generate a magnetic force which tends to operate the switch. The mode of operation, either monostable or bistable, is creating by externally positioning one or two permanent magnets to the capsule.
The reeds used in the capsule for making the switching contacts are to be made of a material that must be carefully chosen in order to meet three requirements:
1) it must be optimally magnetic conducting, PA1 2) it must be optimally electric conducting, PA1 3) it must have an expansion coefficient that is compatible with that of glass.
In practice, the choice of the material for the reeds is a compromise between the said three requirements and can never optimally meet all and each of them.